This proposal will continue the development of an innovative MRI method that can measure the extracellular pH (pHe) of the tumor microenvironment. Our MRI method uses Chemical Exchange Saturation Transfer (CEST) agents that accurately and precisely measure pH. We have used our CEST agents and CEST MRI methods to map tumor pHe in mouse tumor models. We propose to improve the detection sensitivity of CEST MRI to facilitate our pre-clinical studies eventual clinical translation. Our research has strong impact because tumor pHe measurements may be used to predict the effects of weak-base and weak-acid chemotherapies before they are administered to a patient. Tumor pHe measurements may also be used to monitor alkalinizing therapies that can increase survival and decrease metastasis. We propose to establish that our CEST MRI method can measure tumor pHe with sufficient accuracy and precision to impact the choice of cancer therapies. Specific Aim 1: To improve the detection sensitivity of CEST MRI methods and CEST agents that measure tumor pHe. We will optimize the saturation period of the CEST MRI pulse sequence, and improve our paramagnetic CEST agents to have larger chemical shifts. Specific Aim 2: To monitor the effect of alkalinizing therapy using tumor pHe measurements. We hypothesize that monitoring pHe can be used to optimize the dose of an alkalinizing therapy to raise tumor pHe without affecting kidney pHe. Specific Aim 3: To quantitatively predict chemotherapeutic effects with tumor pHe measurements. We hypothesize that a single tumor pHe measurement before initiating chemotherapy can quantitatively predict the therapeutic effect on tumor growth. Our deliverable is a clinically-translatable imaging method that can profoundly impact personalized medicine. By quantitatively measuring the acidity of a solid tumor, a physician may then select the weak-base or weak- acid chemotherapy that should have a better therapeutic effect for that individual patient. By monitoring the pHe of the tumor and kidney, a physician may adjust an alkalinizing treatment dosage to provide an optimal therapeutic effect for an individual patient. Although our research focuses on breast cancer, our methodology can impact personalized medicine for many other cancers and other pathologies.